120/208 triplex aerial feeder

[hoots]

My brother-in-law is needing to string up a 3phase, 150amp, 120/208 service 750' away for his irrigation pump and lighting/120 power. After looking at table 310.68 in the nec 2008 code, it's saying that 1/0 aluminum (triplexed in air) is good for 170 amps. Now after doing a quick calculation of voltage drop, i came up with around 30 volts dropped at that length. just wondering if this sounds right, and if it would be more efficient to either stay with the 1/0 that's rated for the ampacity needed, and get a transformer to boost the voltage up another 30 volts. or to go with lets say 4/0 that would greatly reduce the amount of v.d. I'm worried about weight and span with the 4/0 in the air...Thanks!

 

[augie47]

As long as you don't exceed the recommended span length, I would not worry about the weight.
You might compare price and availability for paralleling the 1/0s

 

[GoldDigger]

My brother-in-law is needing to string up a 3phase, 150amp, 120/208 service 750' away for his irrigation pump and lighting/120 power. After looking at table 310.68 in the nec 2008 code, it's saying that 1/0 aluminum (triplexed in air) is good for 170 amps. Now after doing a quick calculation of voltage drop, i came up with around 30 volts dropped at that length. just wondering if this sounds right, and if it would be more efficient to either stay with the 1/0 that's rated for the ampacity needed, and get a transformer to boost the voltage up another 30 volts. or to go with lets say 4/0 that would greatly reduce the amount of v.d. I'm worried about weight and span with the 4/0 in the air...Thanks!

If the 30 volt VD is for the pump FLA only, I would definitely be worried about the effect of the even higher voltage drop on the lighting during pump starting. A boost transformer will do nothing for the voltage dip during starting and may also lead to too high a voltage when the pump is off.

 

[Smart $]

... After looking at table 310.68 in the nec 2008 code, it's saying that 1/0 aluminum (triplexed in air) is good for 170 amps. ...

You will have to transition at each end to a Table 310.15(B)(16) size for 75°C terminations... 1/0 copper or 3/0 aluminum.

 

[Fnewman]

The problem with trying to use a transformer to overcome voltage drop is that the drop varies with load, so at times it could be anything less than the 30 calculated (or more if the load was increased in the future).

 

[kwired]

go with a 480 volt feed and 480 volt motor on the pump,

Conductors can be smaller, motor controls can be smaller, higher voltage/ less current means voltage drop is much less of an issue.

If you have a 150 amp load @ 208 volts (likely a 50 or 60 HP motor) it is likely worth going that route,

you can thank me later

 

[Smart $]

go with a 480 volt feed and 480 volt motor on the pump, ...

Unless he gets the PoCo to install a 480V service, he's still looking at transformers on both the supply and load ends and the same reduced wire size.

However, the load-end transformer can be much smaller.

 

[kwired]

Unless he gets the PoCo to install a 480V service, he's still looking at transformers on both the supply and load ends and the same reduced wire size.

However, the load-end transformer can be much smaller.

If he goes with a 480 volt motor there is no need for load end transformer. Also will be smaller motor starter, fuse/switch/breaker for 480 volt then for 208 volt.

Around here the POCO to get better rate structures you need to put that irrigation on it's own service anyway. With the better rate structurs comes the ability of the POCO to remotely turn your load off during peak demand periods. How long they may be able to turn it off depends on which rate you want to subscribe to. If you don't want it turned off at all you are on a higher rate. If you want lowest rate possible, you may have times where they can turn you off for up to 12 hours in a day. Being off for the full 12 hours usually only happens in the hottest, driest years though. You will be turned off at times pretty much every year though.

 

[Smart $]

If he goes with a 480 volt motor there is no need for load end transformer. ...

Only if he goes with no 120V lighting & power mentioned in the OP.

 

[kwired]

Only if he goes with no 120V lighting & power mentioned in the OP.

Seems if a 2-5 kVA single phase transformer is needed for 120 volt loads, that would cost a lot less then the possibly needed 75 kVA transformer to step voltage for all the load back to 208/120.

Owner likely isn't just going to add 150 amps to what is existing either unless he already has a large operation of some kind with some extra capacity built into it, so either have POCO give him a separate 480 volt service for this or still spend some $$ upgrading the existing service to handle it. Unless you have a difficult POCO, the 480 volt service probably wins the pro/con list in most cases.